Back problems are a widespread disease, with which probably every person makes acquaintance in the course of his life. However, it is often not the bony components of the spine that cause problems, but the intervertebral discs, also called Disci intervertebrales.
What are the intervertebral discs?
Schematic anatomical representation of the vertebrae and the intervertebral disc, as well as the pinched nerve. Click to enlarge. A simplified definition of the intervertebral discs might be that they are cushions of water that act as natural shock absorbers to absorb the shock of walking. The intervertebral discs are thus fibrocartilaginous, flexible connections between the individual vertebral elements. The human spine contains 23 intervertebral discs. They are located between the vertebral bodies and contribute to mobility and shock elasticity. The intervertebral discs occupy about 25% of the total length of the spine.
Anatomy and structure
A basic understanding of the anatomy and structure of intervertebral discs is important to explain their benefits. They contain two different types of tissue: In the center is a gelatinous core of cell-poor tissue called the nucleus pulposus, which is surrounded on the outside by a fibrous ring called the annulus fibrosus. The annulus fibrosus consists of fibrocartilage, i.e., a tight collagenous connective tissue with embedded cartilage cells. The collagenous fibers are arranged in concentric lamellae, resulting in an intersecting, opposing pattern that serves to optimize force transmission. The outer lamellae radiate into the marginal ridges of the vertebral bodies, while the inner lamellae are connected to the cartilage-covered top plates of the vertebrae. Towards the middle, the fibrocartilage merges smoothly into the gelatinous substance of the nucleus pulposus. This consists largely of glycosaminoglycans and has a high water-binding capacity. It develops an outward swelling pressure, causing the fibrous ring to tighten. Under the load of the upper body while standing or sitting, water is squeezed out of the gelatinous core during the course of the day, and the height of the intervertebral discs decreases. As a result, the height of the body in the evening can be up to 2.5 cm less than in the morning. When lying down, the gelatinous core absorbs water again. This inflow and outflow of fluid simultaneously provides nutrition to the disc, which contains very few blood vessels.
Functions and tasks
The function and tasks of the intervertebral discs can best be illustrated by considering the pressure conditions in the spine. It supports the weight of the upper body, which means that the intervertebral discs are subjected to vertical pressure, which they distribute evenly among the cover plates of the adjacent vertebral bodies. During the impacts that occur when walking, the water-rich gelatinous core cannot be compressed, so it expands laterally toward the fibrous ring, putting it in tension. However, fibrocartilage is not a very stretchable tissue, so the effect of this “shock absorber” is small. In addition to cushioning shocks, the intervertebral discs have the task of limiting the movements of adjacent vertebrae. They provide stability in the spine by limiting rotational motion and forward, backward, or sideways tilting motion between vertebrae.
Diseases
Many diseases and conditions that can occur in relation to the intervertebral discs are due to non-physiological stresses. However, genetic causes or wear and tear over the course of a lifetime can also affect the function of the intervertebral discs. In the cell-poor gelatinous core, the very low metabolic rate leads to molecular changes as early as the third decade of life, which reduce the water-binding capacity. The swelling pressure in the nucleus is reduced, and the fibrous ring is no longer taut. As a result, the disc is less able to absorb shocks and limit sliding movements between the vertebrae. It also remains permanently flat, putting excessive stress on the vertebral arch joints. This can result in spondylarthrosis, which means that the joint cartilage is abraded and new bone tissue proliferates. A widely known complaint is also the herniated disc (disc prolapse). Abnormal stress causes tears in the fibrous ring and parts of the gelatinous core emerge. This tissue often penetrates into the spinal canal and presses the spinal nerve running there against the vertebral arch joint. This can result not only in pain but also in sensory or motor deficits.The transition between the cervical and thoracic spine and between the lumbar spine and sacrum are particularly susceptible. There, those spinal nerves are irritated that innervate the leg via the sciatic nerve. Often, the back muscles tense up to widen the narrowed spinal canal or immobilize the affected motion segment, resulting in painful “lumbago.”
Typical and common conditions
- Herniated disc
- Disc degeneration
- Scheuermann’s disease (Scheuermann’s disease)
- Hollow back (hyperlordosis)
- Failed back surgery syndrome (postdisectomy syndrome).